Method and device for controlling a piezo-actuator

ABSTRACT

A method of triggering a piezoelectric actuator which controls the injection of fuel into the combustion chamber of an internal combustion engine via a valve is described in which the operating situation of the engine is determined and the derivative with respect to time of the voltage, which can be picked off at the piezoelectric actuator, is selected as a function of the operating situation. Furthermore, a control unit for controlling a fuel injection system is described, in which a piezoelectric element is triggered so that the derivative with respect to time of the voltage, which can be picked off at the piezoelectric actuator, is adjusted to the operating situation of the engine. Additionally described is a fuel injection system, having at least one piezoelectric actuator which is triggered accordingly.

FIELD OF THE INVENTION

The present invention relates to a method, a control unit, and a fuelinjection system, respectively, where a piezoelectric actuator iselectrically recharged by the application of an electric current inorder to change its length.

BACKGROUND INFORMATION

Such a method, in which the derivative with respect to time of thevoltage applied to the piezoelectric actuator is changed within acharging or discharging operation, is discussed in German PublishedPatent Application No. 199 21 456.

SUMMARY OF THE INVENTION

The exemplary method and the exemplary devices according to the presentinvention may lower the noise emissions of the injection system in thoseoperating situations where they are significantly influenced by thetriggering of the piezoelectric actuators utilized. In addition, incommon rail injection systems in particular, the system behavior, i.e.,the accuracy of triggering, as well as the metering of the injectedquantities may remain unaffected, such as, for exampe, at high railpressures, i.e., that even at high rotational speeds or high loads onthe internal combustion engine the required timing tolerances withrespect to triggering, as well as the accuracy of the metered quantity,may be complied with.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows two voltage-time diagrams.

FIG. 2 shows a flow diagram.

FIG. 3 shows a block diagram.

FIG. 4 shows an additional block diagram.

DETAILED DESCRIPTION

FIG. 1 a shows a voltage-time diagram. It shows the variation of voltageover time across a piezoelectric actuator which controls the injectionof fuel into the combustion chamber of an internal combustion engine viaa valve. Two standard triggering characteristics are illustrated. In thefirst triggering, voltage U is linearly increased within charging time 1from zero to a value ΔU1 which is maintained for a certain time (e.g.,ΔU1=200 V). During subsequent discharging time 2, the voltage applied tothe piezoelectric actuator is again linearly reduced to zero. The secondtriggering has an intermediate level ΔU2 (e.g., ΔU2=100 V), to which thevoltage is initially increased within charging time 3. After reachingthis voltage level, the voltage is increased by the difference value ΔU3(e.g., ΔU3=100 V) within additional charging time 4, to be onlysubsequently reduced in two steps to the value zero within dischargingtimes 5 and 6. FIG. 1 b shows similar voltage characteristics havingidentical voltage levels ΔU1 and ΔU2, respectively. However, thecharging times and discharging times 7, 8, 9, 11, 12, and 13 are longerthan the charging times and discharging times 1 through 6 in FIG. 1 a.The absolute value of the derivatives with respect to time of thevoltage characteristics in the charging times and discharging times istherefore less than in FIG. 1 a. Any triggering characteristics that maybe represented by broken lines may be supported and the abovedescription is appropriately applicable.

In injection systems having piezoelectric actuators, a control valvewhich controls the movement of the nozzle needle may not be triggereddirectly, but via a hydraulic coupler, as discussed in German PublishedPatent Application No. 197 32 802, for example. This coupler hasessentially two functions: First, it reinforces the lift of thepiezoelectric actuator and second, it decouples the control valve fromthe static thermal expansion of the actuator. The triggering voltagerequired for accurate positioning of the control valve and thus forimplementing a desired injection may be heavily dependent on the fuelpressure and, in a common rail system, on the rail pressure of the fuel.This may be explained by the feature that the control valve worksagainst or with the rail pressure, depending on the switching directionof the valve. The derivative with respect to time of the triggeringvoltage may be selected so that the charging time and discharging timecorrespond exactly to the time constant of the mechanical system. Thevibration induced in the system may be minimized in this case. Fordifferent reasons, it may be desirable to keep the charging time anddischarging time as short as possible, in particular to implementtriggering periods as short as possible, in order to supply the smallestinjected quantities, which may be important at high rail pressures.

On the other hand, the noise emission may increase notably with thegradient, i.e., the derivative with respect to time of the voltagesince, due to the high speed of the actuator movement, the control valveis also moved with similar speed. This effect may be interfering incertain operating situations of the engine. In this connection, theexpression “operating situation” is not to be understood as a certainperiod of time within a triggering of the piezoelectric actuator, butrather as the operating condition, generally present through severalinjection cycles, such as idling, for example, which may becharacterized by small load and low rotational speed. Triggeringaccording to FIG. 1 a may be used in normal driving operation underload, while in the operating situation “idling”, a triggering accordingto FIG. 1 b having a flatter triggering gradient may achieve a reductionin noise emission, particularly here where the noise caused bytriggering of the injection system is noticeable compared to othervehicle noises.

FIG. 2 illustrates the procedure of triggering of a piezoelectricactuator which, in a common rail injector for example, may control theinjection of diesel fuel into the combustion chamber of the dieselengine. After switching on 10 of the engine, i.e., the injection system,it is first verified in query 20 whether a charging/dischargingoperation is requested. If this is the case, the operating condition ofthe engine is determined (process step 30). The operating condition ofthe engine may be characterized by the rotational speed and/or the loadon the engine and/or by the fuel pressure in the injection system.Further characterizing variables may be the temperature of thepiezoelectric actuator, the temperature of the fuel, or othercharacteristic data. In subsequent process step 40, the setpoint of thederivative with respect to time of the voltage which is to be applied tothe piezoelectric actuator is determined as a function of the operatingcondition of the engine. The gradient setpoint is set here so that thenoise development due to the movement of mechanical components may beminimized while the functionality of the injection system is preserved.When certain threshold values of the rotational speed, the load torque,and/or the rail pressure are reached here (e.g., rotational speed<2000rpm, the load is less than 10% of the maximum load and the rail pressureis below 500 bar), then a smooth transition of the gradient setpoint, incomparison to “normal operation,” is implemented, so that below thethreshold values mentioned, the derivative with respect to time of thevoltage to be applied changes over continuously to smaller values. Thecharging time or the discharging time varies typically (e.g., at 50% ofthe maximum load) between 80 μs and 100 μs, while it assumes valuesbetween 100 μs and 150 μs below the threshold values.

In subsequent query 50, it is checked whether it is the first request ofthe injection system after switching on. If yes, a driver signal iscalculated for a driver which triggers a charging/dischargingarrangement to be applied to the piezoelectric actuator. The driversignal is calculated here so that a sufficient electric current is fedto the piezoelectric actuator in order to achieve the determinedsetpoint of the derivative with respect to time or thecharging/discharging time of the voltage to be applied. In additionalstep 80, the driver that triggers the charging/discharging arrangementis triggered until the final value of the electric voltage across thepiezoelectric actuator is reached. In an additional step 90, the actualvalue of time is determined, which was required to charge or dischargethe piezoelectric actuator to the voltage to be achieved. The programsubsequently returns to query 20.

If in query 50 the result is “No,” then the system deviation, i.e., thedeviation of the last actual value of the time needed for therecharging, from the calculated setpoint, is determined and is takeninto account in subsequent process step 70 for calculating the driversignal for the next recharging of the piezoelectric actuator.

The change in triggering only in certain operating points, such asidling (characterized above by the threshold values mentioned), may beentirely sufficient, since, due to triggering, only in these points maythe noise, imitated by the injector, significantly influence the overallnoise of the drive unit. In partial load or full load operation,however, the overall noise may be far dominated by the combustion noise.The present invention is based on the idea that in order to implement amore constant charging/discharging time in the range of the system time,the triggering gradients, i.e., the charging/discharging times are notchanged, as previously, as a function of the voltage, but are switchedover to a flatter gradient in certain operating situations, inparticular during idling. In doing so, the noise emission may besignificantly reduced. The rail pressure may also be relatively lowduring idling, so that even during longer charging/discharging times,the smallest injected quantities may be implemented and the narrowtolerances to be adhered to with regard to the injected quantities maybe ensured.

Alternatively to a smooth transition of the gradient or she timesetpoint between normal operation and idling, a hard switch-over tosmaller gradients may also be provided when one or several of thethreshold values fall below a certain value.

FIG. 3 shows a control unit 200 which is connected to a driver 120 andcharging/discharging arrangement 110. The control unit has a monitoringunit 150 which is supplied with operating condition variables 210. Theseoperating condition variables are the rotational speed, the load torque,the rail pressure, and/or the temperature of the piezoelectric actuator,and/or the fuel temperature, and/or other parameters. Monitoring unit150 determines the setpoints for the charging/discharging times and thecharging/discharging gradients and transmits these to logic circuit 130.Logic circuit 130 is connected to an actual value detecting unit 140,which, as illustrated in FIG. 3, may be integrated into the controlunit, but may also be arranged separately in the immediate proximity ofcharging/discharging arrangement 110. Actual value detecting unit 140 isconnected to charging/discharging arrangement 110. Logic circuit 130 mayreceive a request signal from higher-level engine control units (notshown) via line 220. Logic circuit 130 is connected to a driver 120which, in turn, is interconnected with charging/discharging arrangement110 which applies a voltage to piezoelectric actuator 100 as a functionof time.

The setpoint for the charging/discharging time is determined inmonitoring unit 150, taking into consideration the variables rotationalspeed, load, and rail pressure, and the monitoring unit transmits thedetermined value to logic circuit 130. Upon request, logic circuit 130calculates a driver signal via signal line 220 taking into considerationthe actual value of the charging/discharging time or thecharging/discharging gradient measured by actual value detection unit140. Logic circuit 130 conveys the driver signal to driver 120 whichthen triggers charging/discharging means 110 in order to implement thevoltage gradients to be achieved across piezoelectric actuator 100.

To regulate the control gradients during the recharging phases,variables other than rotational speed load and/or rail pressure may bealternatively used for determining the operating condition of the engineand/or the injection system.

FIG. 4 shows a component 131 of logic circuit 130 in the form of a blockdiagram. The actual value detected by actual value detection unit 140and the setpoint calculated by monitoring unit 150 are fed to a summingnode 255 via lines 250 and 260, respectively. The summing nodecalculates the system deviation, i.e., the difference between thesetpoint and the actual value and feeds this difference to PI regulator270, i.e., a proportional amplifier, which is connected in parallel toan integrator. The output of PI regulator 270 is connected to a secondsumming node 275 which adds the output value of the PI regulator to thesetpoint from monitoring unit 150. Prior to or following the rechargingprocedure to be calculated, the voltage levels are fed via lines 280 or290 to a third summing node 285 which calculates their difference andfeeds it to multiplier 295 which, in turn, calculates the chargerequired for the recharging procedure from the difference and the valueof the capacitance of the piezoelectric actuator fed via line 300.Divider 305 divides the value of the electric charge, obtained frommultiplier 295, by the value of the charging/discharging time obtainedfrom summing node 275, so that the information about the current valuerequired for the recharging procedure at the piezoelectric actuator maybe picked off at output 310 of divider 305. Output 310 of divider 305 isconnected to driver 120 and is available to it for triggeringcharging/discharging means 110 (see FIG. 3). Lines 280, 290, and 300 areconnected either to storage elements in which the voltage andcapacitance values to be retrieved are stored, or they are connected toseparate circuit elements (not shown) which recalculate or define thevoltage and capacitance values, as a function of the triggering demandand switching condition.

Component 131 implements the process steps illustrated in FIG. 2. Thecharging and discharging time are regulated by a PI regulator, thedifference between the voltage levels to be bridged, and the actuatorcapacitance of the associated charging and discharging current beingdetermined.

1. A method of triggering a piezoelectric actuator which controls aninjection of fuel into a combustion chamber of an internal combustionengine via a valve, the method comprising: applying an electric currentto one of at least partially charge and discharge the piezoelectricactuator and to change a length of the piezoelectric actuator;determining an operating situation of the internal combustion engine;and selecting, as a function of the operating situation, a derivativewith respect to time of a voltage across the piezoelectric actuatorduring a charging/discharging time; wherein the operating situation isdefined by at least one of a rotational speed and a fuel pressure in aninjection system of the internal combustion engine.
 2. The method ofclaim 1, wherein the injection system is a common rail system, and thefuel pressure is a pressure of the fuel in a rail of the common railsystem.
 3. The method of claim 1, further comprising: reducing, if theoperational situation is a low fuel pressure, the derivative withrespect to time compared to the operating situation of at least one of ahigher rotational speed and a higher fuel pressure.
 4. The method ofclaim 3, wherein the derivative with respect to time is reduced duringan idling of the internal combustion engine.
 5. The method of claim 1,further comprising: adjusting an absolute value of the electric currentapplied to the piezoelectric actuator during charging and discharging,depending on the derivative with respect to time to be achieved.
 6. Themethod of claim 1, wherein the internal combustion engine is a dieselengine.
 7. A control unit for controlling a motor vehicle injectionsystem, comprising: at least one piezoelectric actuator to inject fuelinto a combustion chamber of an internal combustion engine via a valve;an electric current applied to the at least one piezoelectric actuatorto one of at least partially charge and discharge the at least onepiezoelectric actuator and to change a length of the at least onepiezoelectric actuator; and a monitoring unit to determine an operatingsituation of the internal combustion engine so that a derivative withrespect to time of a voltage across the at least one piezoelectricactuator during a charging/discharging time is selectable as a functionof the operating situation; wherein the operating situation is definedby at least one of a rotational speed and a fuel pressure in aninjection system of the internal combustion engine.
 8. The control unitof claim 7, wherein the fuel pressure is given by a pressure of the fuelin a rail of a common rail system of the internal combustion engine. 9.The control unit of claim 7, wherein, upon at least one of a lowrotational speed and a low fuel pressure, the monitoring unit isconfigured to reduce the derivative with respect to time as compared tooperating situations of at least one of a higher rotational speed and ahigher fuel pressure.
 10. The control unit of claim 9, wherein thederivative with respect to time is reduced during an idling of theinternal combustion engine.
 11. The control unit of claim 7, wherein anabsolute value of the electric current applied to the piezoelectricactuator during charging and discharging, respectively, is adjusted as afunction of the derivative with respect to time to be achieved.
 12. Thecontrol unit of claim 7, wherein the internal combustion engine is adiesel engine.
 13. A fuel injection system comprising: at least onepiezoelectric actuator for injecting fuel into a combustion chamber ofan internal combustion engine via a valve; an electric current appliedto the piezoelectric actuator to at least one of at least partiallycharging and discharging the at least one piezoelectric actuator and tochange a length of the at least one piezoelectric actuator; and acontrol unit to determine an operating situation so that a derivativewith respect to time of a voltage across the at least one piezoelectricactuator during a charging/discharging time is selectable as a functionof the operating situation; wherein the operating situation is definedby at least one of a rotational speed and a fuel pressure in aninjection system of the internal combustion engine.